When applying a coating of solder to surfaces to be soldered, it is necessary to remove oxides, otherwise wetting of the surface by solder may not occur. The term "wetting" refers to solder bonding to metal surfaces. If solder does not bond to metal surfaces then wetting has not occurred. This is unacceptable, particularly on component leads to be solder coated and on printed wiring boards having components to be soldered thereon. In many cases, the components have pins that extend through holes and it is necessary to ensure that the solder "wicks" up through the holes and wets both the exterior surfaces of the pins and the inside surfaces of the holes to ensure a proper soldered contact.
In the past one applied flux to the surfaces to be soldered and whereas flux dissolves the oxides that are present on the surface, flux and oxide residues may be formed in the process, and in the case of solder dipping or wave soldering, these residues can collect on the components or in the solder reservoir which can be harmful to the solder process, to the reliability of the electronic components or circuit assemblies, and in some cases to the environment.
It has been found that if soldering occurs on good solderable surfaces within an inert atmosphere and on occasions with a small quantity of hot reducing gas such as hydrogen, that flux is not needed. Good wetting occurs and reliable soldering can result. However, oxide deposits that have built up on metal surfaces require to be removed prior to soldering to ensure proper solder wetting.
In a paper published in June, 1987, entitled "New Copper Surface Treatment for Polyimide Multilayer Printed Wiring Boards" by Haruo Akahoshi and Yoshihiro Suzuki, is disclosed a reduction treatment for a surface oxide layer to give a metallic copper surface with no changes in its morphology. This conversion was shown to occur by means of wet chemical or electrochemical reduction. Varying the current density in the electrochemical reaction changed the surface structure. It was reported that a higher current density retained the original surface structure while low current density changed the original surface structure. The authors reported that if the original surface structure is retained, the adhesion characteristics are identical to those of the untreated surfaces.
Electrochemical reduction for copper and tin-lead surfaces is disclosed in an article by Tench and Anderson entitled "Electrochemical Assessment of Sn-Ph Solderability" published August, 1990, in Plating and Surface Finishing. The electrochemical reduction was performed in a sodium borate solution under an inert gas utilizing a Pt counter electrode and a reference saturated calomel electrode (SCE).
Reduction of the oxides on metallic surfaces for improvements in multilayer board adhesion using electrochemical reduction is disclosed in the above article. Unlike chemical treatment which uses a flux to dissolve oxides, electrochemical reduction treatment does not dissolve the oxides but treats the surfaces electrochemically. The elements having metallic oxides on the surfaces to be electrochemically treated have the liquid applied to these surfaces, usually in a dip tank somewhat similar to an electroplating bath. The liquid reduces the oxides wherein the treated metal surfaces are cleaned of oxides. There are substantially no residues remaining on the treated metallic surfaces by this reduction process as compared to the application of fluxes.
Electrochemical reduction, sometimes known as reverse plating has conventionally been a batch type operation such as dipping in a tank containing treatment liquid. Spraying, while used for wet chemical spraying, generally is not appropriate with wet batch processing as applied in board fabrication shops. Spraying cannot be used for electrochemical reduction as current will not flow in a spray, unless a continuous flood is used. Continuous dipping requires a conveyor which travels down into a tank and then up out of the tank. Such a non-in-line conveyor becomes complex. Thus there is a requirement to flood components such as circuit boards and the like which is simple and does not cause atomization of the treatment liquid.
It is an aim of the present invention to provide a novel method and apparatus for generously flooding an electrochemical liquid, or in some cases an electroless liquid, onto metallic surfaces of an element to be treated for soldering to remove oxides by reduction rather than by fluxing, without dipping or spraying.